1. Field of the Invention
The present invention relates to a hot strip rolling plant which performs a series of operations from continuous casting to finish rolling in a through line, and more particularly to a hot strip rolling plant and method directly combined with continuous casting, which can realize small-scale production of products with a small-scale facility.
2. Description of the Prior Arts
As described in "Recent Hot Strip Manufacture Techniques in Japan", (published by Japan Steel Association (Incorporated Body), Aug. 10, 1987), pp. 6-10 and p. 176, for example, a typical hot strip rolling plant (hereinafter referred to as a hot strip mill) is conventionally large-scaled such that a slab being 200 mm thick is rolled by one or plural roughing mills into a bar with a thickness of 20 to 40 mm, which bar is then rolled by tandem finishing mills of 6 to 7 stands. Such a hot strip mill provides a yield of 3 to 4 million tons per year and is adapted for mass production (hereinafter referred to as first prior art). A larger-scale hot strip mill provides a yield of 3 to 6 million tons per year.
Hitherto, there has naturally existed a demand for a small-scale production system in which the production scale is reduced and the plant size is also reduced correspondingly. Recent generation of iron scraps in a great deal amount has set importance on recycling of those scraps, and the concept that small-scale hot strip mills should be dispersedly installed for a convenience in collecting the scraps rather than centralizing large-scale hot strip mills, has prevailed in the world. Such a small-scale hot strip mill is simply called "mini hot". Thus, needs for optimum mini hots have become more keen.
As described in "Hitachi Hyoron Vol. 70, No. 6", (Jun. 25, 1988), pp. 67-72, for example, there is known a small-scale production system called a Steckel mill which comprises one reversible roughing mill and coiler furnaces installed upstream and downstream of the reversible mill. The steckel mill is widely employed for rolling steel strips which are less susceptible to scales, such as stainless steel plates (hereinafter referred to as second prior art).
Meanwhile, although a plate steel (hereinafter referred to as a slab) forwarded to the roughing mill is generally about 200 mm thick, recent development of a thin slab continuous casting process has succeeded in manufacturing a slab with a thickness not larger than 80 mm, e.g., about 50 mm. In some cases using such a slab, no rough rolling mills are employed and the hot strip mill is made up by a train of finishing mills only.
For example, "Ein Jahr Betriebserfahrung mir der CSP-Anlarge fur Warmbereitband bei Nucor Steel", (Stahl u. Eisen 111 (1991) Nr. 1) describes a hot strip mill utilizing the thin slab continuous casting process which intends to realize a mini hot with no roughing mills by dividing a thin slab into the length of about 40 m and rolling the divided slab after heating it and holding a heated condition. This hot strip mill employs a conventional finishing mill train comprising mills of 5 to 7 stands, shows a high rolling speed not less than 300 m/minute for the finish thickness of 2.5 mm and not less than 600 m/minute for the finish thickness of 1.6 mm on the delivery side of the finishing mill train, and has a plant length of about 250 mm (hereinafter referred to as third prior art).
Further, "First Mini Mill with ISP" intends to realize a mini hot in which a thin slab being 40 mm thick after reduction in a non-solidified state is rolled by a train of 3-stand roughing mills, an induction heater, an intermediate coiler, and 4-stand finishing mills. In this hot strip mill, a bar rolled by the roughing mills to have a thickness of 15 mm is once reeled into the form of a coil, and the coiled strip is batch-supplied to the finishing mills so that a continuous casting machine and the finishing mills are separated from each other to compensate for a large difference in speed therebetween and to maintain the strip at a required temperature. Since this hot strip mill also employs a conventional finishing mill train, it shows a high rolling speed not less than 500 m/minute on the delivery side of the finishing mill train for maintaining the strip at a required temperature, and has a plant length not less than 150 mm in spite of using the intermediate coiler (hereinafter referred to as fourth prior art).